Zemanek, U.S. Pat. No. 3,369,626 discloses an ultrasonic tool for use in scanning the inner surface of an open well borehole or of casing in a borehole. The tool, which is commercially known as the "borehole televiewer", creates a high resolution picture of the inner surface under investigation. The borehole televiewer is used to "see" the inner surface under investigation through drilling mud or other borehole fluids. In an open borehole, the borehole televiewer provides a picture of the formations surrounding the borehole. In a cased borehole, the borehole televiewer provides a picture of the inner surface of the casing, which can be used to determine the condition of the inner surface.
The borehole televiewer uses a rotating ultrasonic transducer. The transducer serves as a transmitter, to generate acoustic waveforms, and a receiver, to receive the acoustic return. The acoustic return is caused by the reflection of the generated acoustic waveform from the inner surface under investigation. The acoustic return has two measured parameters, the time of travel of the acoustic return and the amplitude, which give an indication of the condition of the investigated surface.
The transducer rotates about three revolutions per second, is pulsed about 500 times per revolution, and is pulled up the borehole at a speed of about 5 feet per second. The ultrasonic transducer spot size, the rotational speed, the pulse repetition rate, and the vertical speed combine to provide full coverage of the investigated inner surface, resulting in high areal resolution of the inner surface.
Because of the large amount of information generated by the borehole televiewer and because the frequency of the acoustic waveforms is high, the bandwidth of the information is high. Unfortunately, the logging cable, which utilizes electrical conductors to connect the borehole televiewer to the surface equipment, has a limited bandwidth, thereby limiting the amount of information that can be transmitted over the logging cable. Fiber optic logging cables have the necessary bandwidth, but are too expensive and too easily damaged for general use.
Because of the limited bandwidth of the logging cable, the borehole televiewer is limited to transmitting the envelope of the acoustic return over the logging cable to the surface electronics.
Improvements in ultrasonic apparatuses allow investigations beyond the inner surface of the borehole. For example, as shown in Havira, U. S. Pat. No. 4,255,798, ultrasonic apparatuses are used to measure casing wall thickness and to evaluate the bond between cement and casing in a borehole.
It is thus desirable to transmit more information uphole than is contained in the envelope of the acoustic return. Although real-time signal processing is occurring downhole in the ultrasonic apparatus, it is desirable to record with the surface equipment the actual acoustic return. As more sophisticated methods of processing the acoustic returns are developed, these methods can be used to reprocess existing data, if the actual acoustic returns are recorded.